University of Groningen
Absolute photoluminescence quantum yield enhancement of poly(2-methoxy 5-[2'ethylhexyloxy]-p-phenylenevinylene)
Marchioni, F.; Chiechi, Ryan; Patil, S.; Wudl, F.; Chen, Y.; Shinar, J.
Published in:
Applied Physics Letters
DOI:
10.1063/1.2335365
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Marchioni, F., Chiechi, R., Patil, S., Wudl, F., Chen, Y., & Shinar, J. (2006). Absolute photoluminescence
quantum yield enhancement of poly(2-methoxy 5-[2'-ethylhexyloxy]-p-phenylenevinylene). Applied Physics
Letters, 89(6), 1-3. [061101]. DOI: 10.1063/1.2335365
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APPLIED PHYSICS LETTERS 89, 061101 共2006兲
Absolute photoluminescence quantum yield enhancement
of poly„2-methoxy 5-†2⬘-ethylhexyloxy‡-p-phenylenevinylene…
F. Marchioni, R. Chiechi, S. Patil, and F. Wudla兲
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095
and Exotic Materials Institute, University of California, Los Angeles, California 90095
Y. Chen and J. Shinar
Ames Laboratory-U.S. DOE, Iowa State University, Ames, Iowa 50011
and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011
共Received 23 March 2006; accepted 15 June 2006; published online 7 August 2006兲
A twofold absolute photoluminescence quantum yield 共PLQY兲 enhancement of poly共2-methoxy
5-关2⬘-ethylhexyloxy兴-p-phenylenevinylene兲 共MEH-PPV兲 is demonstrated by simple preparation of
films where the polymer is blended with the small organic molecule 7,8,10-triphenylfluoranthene
共3PF兲. The photophysical investigation of this particular energy transfer process was carried out
using steady state absorption/luminescence spectroscopy and optically detected magnetic resonance
techniques. The enhanced PLQY is attributed to direct sensitization of the intrachain emitting state
of MEH-PPV by energy transfer from 3PF. © 2006 American Institute of Physics.
关DOI: 10.1063/1.2335365兴
In the last 15 years the fast growing field of organic light
emitting diodes 共OLEDs兲 has led to the synthesis of numerous new luminescent polymers. An important class of these
polymers consists of poly共p-phenylenevinylene兲 共PPV兲 and
its derivatives such as poly共2-methoxy 5-关2⬘-ethylhexyloxy兴p-phenylenevinylene兲 共MEH-PPV, Fig. 1兲. Their stability, solution processability, and electrical and optical properties
have led to their investigation for use in a wide variety of
applications.1 A fundamental understanding of the excited
state processes in these quasi-one-dimensional materials is a
key component in the optimization and development of new
and more efficient OLEDs.
In this letter a twofold absolute photoluminescence 共PL兲
quantum yield 共PLQY兲 enhancement of MEH-PPV is demonstrated by simple preparation of films where the polymer
is blended with the small organic molecule triphenylfluoranthene 共3PF, Fig. 1兲. The photophysical properties of this system and its behavior are described.
MEH-PPV 共molecular weight of 32 000兲 and 3PF were
both synthesized in our laboratory following the procedure
previously reported.2,3 All the films were spin coated from a
10 mg/ ml chlorobenzene solution onto glass slides. A 20 nm
thick thermally evaporated 3PF film was obtained as previously described.3 UV-visible 共UV-vis兲 absorption spectra
were recorded with an 8453 UV-vis Agilent spectrophotometer and PL and PL excitation spectra were obtained with a
spectrofluorometer Fluorolog Spex FL3-11. Absolute PLQY
measurements were conducted following the procedure described by de Mello et al.,4 using an Ar+ laser as light source
and a homebuilt integrating sphere connected to a calibrated
Ocean Optics spectrometer through an optical fiber.
Figure 2 shows the normalized absorption and emission
spectra of the MEH-PPV and 3PF films. In agreement with
previous studies,5 the spin-coated MEH-PPV film shows the
characteristic absorption band in the visible region with ␭max
around 500 nm, followed by an emission with a slightly
structured band having a maximum at 570 nm; the PLQY of
these films was 8%. The lowest energy absorption band peak
of the evaporated 3PF film is at 375 nm; its fluorescence
peaks at 475 nm and its PLQY was 50%.3
The photophysical properties of films obtained by blending 3PF with MEH-PPV 共2:1 w / w兲 were also investigated.
Whereas the absorption spectra of the blends are essentially
the sum of the spectra of the pristine materials, the emission
properties are strongly affected by the energy transfer between 3PF and MEH-PPV. Figure 3 shows the emission
spectra of pristine MEH-PPV and the blended films excited
at 375 and 500 nm, where 3PF and MEH-PPV, respectively,
preferentially absorb the incident light. Due to the good overlap between the 3PF emission and MEH-PPV absorption
共Fig. 2兲 and the potentially close contact between the dopant
and the polymer in the solid film, the energy transfer process
is very efficient for this system. In fact, when exciting at
375 nm, where 95% of the incident light is absorbed by 3PF
only, the emission is due only to MEH-PPV, whereas the
strong solid state fluorescence of 3PF is totally quenched
共Fig. 3兲.
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
FIG. 1. Structural formulas of MEH-PPV and 3PF.
0003-6951/2006/89共6兲/061101/3/$23.00
89, 061101-1
© 2006 American Institute of Physics
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061101-2
Marchioni et al.
FIG. 2. Absorption and emission spectra of pristine MEH-PPV 共dashed line兲
and 3PF 共solid line兲 films.
An interesting feature emerges when comparing the
shapes of the emission bands of the pristine and blended
materials. The slight increase in vibronic structure and the
width decrease suggest that in the blended films the polymer
chains interact more weakly than in the pristine films. This
effect can be attributed to 3PF molecules that intercalate between adjacent polymer chains, reducing intermolecular interactions and inhibiting the formation of interchain excitations. As noted elsewhere, these interchain species are
characterized by an extended delocalization of the electronic
wave function among two or more chains that often give rise
to a decrease in vibrational resolution and an increase in the
bandwidth of the emission.5–7 In comparing the absolute
PLQY of the blends with that of the pristine material, two
important and unusual photophysical phenomena were
found: 共i兲 an enhancement of the PLQY in the blended films
and 共ii兲 a much higher PLQY when exciting the 3PF at
365 nm than when exciting the MEH-PPV band at 488 nm.
To the best of our knowledge the few examples of efficiency enhancement reported in the literature are all related
to the electroluminescence quantum efficiency in blended
polymer light emitting diodes.8–12 In these cases the phenomenon is mainly attributed to changes in the film morphology
Appl. Phys. Lett. 89, 061101 共2006兲
FIG. 4. Qualitative kinetic scheme of the decay processes in the MEH-PPV–
3PF blended films.
induced by the interactions between the two materials that
form the blend.
The phenomenon that we report in this letter is related to
the change in the PLQY of the material and not to changes in
the devices’ efficiency, thus differing from the previous
papers8–12 for reasons described below. The PLQY enhancement of the blended film 共PLQY= 9 % 兲 relative to that of the
pristine material 共PLQY= 7.5% 兲, when both are excited at
488 nm, is due to the presence of 3PF. The latter prevents the
formation of low fluorescence interchain species. The dramatic enhancement in the PLQY of the blended films excited
at 365 nm 共PLQY= 18% 兲 is, to our knowledge, the first observation of this phenomenon; it is suspected to be due to
direct sensitization from 3PF to the intrachain MEH-PPV
excitation.
Theoretically, in a simple dyad system, where only one
acceptor’s excited state has the right energy to be involved in
the energy transfer process, this phenomenon should not be
observed, even if the energy transfer efficiency is ␩ = 1. In
fact, when the energy transfer involves the same acceptor’s
excited state that can be populated by direct absorption of
light, the PLQY of the system is the product of the efficiencies of energy transfer and radiative emission, so it can only
be, at best, the same PLQY obtained when exciting the fluorophore directly. A different scenario, however, can occur
when the acceptor’s emitting state is not the one that is directly populated by absorption. In this case direct sensitization on this emitting excited state 共process KD*-A* in Fig. 4兲
avoids possible internal relaxation from the upper state to the
ground state 共process KX*-A兲, resulting in a strong enhancement of the overall PLQY. To explain the unusual behavior
we found for the blended film excited at the 3PF absorption
band we propose a kinetic scheme where the donor 共D兲 is the
3PF molecule and the acceptor 共A兲 is the MEH-PPV polymer. Considering the shape comparison between emission
bands of pristine and blended films, it is reasonable to conclude that the MEH-PPV chains in the blended films are
more separated than those in the pristine material. As a result, the equilibrium between the strongly emitting intrachain
excited states and the weakly emitting interchain excited
states 共invoked to explain the photophysical properties of
PPV derivatives兲6 is eliminated.
Irradiating the blended films at 365 nm, the 3PF excited
state is mainly populated and two different paths are pos-
FIG. 3. Emission spectra of pristine MEH-PPV film 共dashed line兲 excited at
500 nm and blended MEH-PPV/3PF films excited at 375 nm 共dotted line兲
and 500 nm 共solid line兲.
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061101-3
Appl. Phys. Lett. 89, 061101 共2006兲
Marchioni et al.
in PLQY obtained as a result of exciting the blended film at
365 nm is due to an efficient and direct sensitization of the
MEH-PPV emitting state, and no exciplexes or other excited
state species are involved or responsible for the emission.
In conclusion, we have demonstrated that blending of
MEH-PPV with 3PF can result in a twofold increase in the
PLQY of MEH-PPV, without a noticeable change in its
emission color. Thus, the efficiency of solution-processed
MEH-PPV-based OLEDs can be improved by doping the
MEH-PPV with 3PF. We concluded that this enhanced
PLQY is due to direct sensitization of the intrachain emitting
state in MEH-PPV by energy transfer from 3PF.
FIG. 5. PL excitation 共dashed line, ␭em = 575 nm兲 and absorption spectra
共solid line兲 of the same MEH-PPV–3PF blended films.
sible: energy transfer to one of the excited states centered on
MEH-PPV 共X* , A*兲 or normal radiative and nonradiative decays from the excited 3PF 共D*兲 state to the ground state. As
previously mentioned, no 3PF luminescence was observed in
the blended films, indicating that the sensitization process is
highly efficient. This unusual behavior can be explained by
considering that through sensitization the excited state A* is
populated directly 共process KD*-A*兲, without passing through
X* and thus not losing in this way the fraction of the excited
states that decays nonradiatively to the ground state through
the process KX*-A. This more efficient sensitization mechanism is supported by the PL excitation spectrum, which is
shown in Fig. 5. In fact, with a film that absorbs the same
quantity of light in the PPV band as in the 3PF band, the
excitation spectrum is more intense in the 3PF absorption
band than in the PPV. This proves that the number of emitted
photons resulting from absorption by 3PF is higher than that
obtained from direct absorption by MEH-PPV. To provide
evidence for the origin of this high PLQY band, optically
detected magnetic resonance 共ODMR兲 measurements13 were
conducted on both pristine and blended samples excited at
two different wavelengths. These measurements did not
show any remarkable difference between the ODMRs of
pristine and doped samples. This confirms that the increase
The UCLA group gratefully acknowledges financial support from the Air Force Office of Scientific Research through
F49620-03-1-0101 and the National Science Foundation
through DMR-0209651 and DGE-0114443. Ames Laboratory is operated by Iowa State University for the U.S. Department of Energy under Contract No. W-7405-ENG-82.
The work at Ames was supported by the Director for Energy
Research, Office of Basic Energy Science.
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